1. Field of the Invention
The present invention relates to circuits for power indicators, and more particularly to a circuit for a power indicator in a computer.
2. Description of Related Art
A computer system is composed of hardware and software. The hardware typically includes a motherboard, an optical disk drive, a hard disk drive, a memory, a network card, and so on. When the computer system is running, it is necessary to know working statuses of the hardware. So, indicator lights are used to show the working statues of the hardware, and corresponding drive circuits for driving these indicator lights are combined in the computer system.
Referring to FIG. 1, a typical circuit for a power indicator includes a light emitting diode 10′, a P-channel MOSFET (metal oxide semiconductor field effect transistor) 20′, a resistor 30′, and a controlling circuit 40′. A positive terminal of the light emitting diode 10′ is connected to a power source V0 via the resistor 30′ and a negative terminal of the light emitting diode 10′ is connected to a drain of the P-channel MOSFET 20′. The controlling circuit 40′ is connected to a gate of the P-channel MOSFET 20′. A source of the P-channel MOSFET 20′ is connected to a ground. According to working statuses of a computer, the controlling circuit 40′ sends a controlling signal S0 to the gate of the P-channel MOSFET 20′. For example, when the hardware is writing data, the controlling circuit 40′ sends a controlling signal S0 with high level to turn on the MOSFET 20′ so that the light emitting diode 10′ is turned on to thereby show the working statuses of writing data of the hardware.
When the computer is on and working normally, the level of the controlling signal S0 is set by the controlling circuit 40′ according to working statuses of the computer. The light emitting diode 10′ can correctly show the working statuses of the computer. However, when the computer is down and needs to be restarted or rebooted, the controlling circuit 40′ may send no controlling signal to enable the P-channel MOSFET 20′ to be turned off, and the light emitting diode 10′ may remain lit giving a false indication.